The present disclosure relates to a battery pack that is applicable to an electric tool such as an electric drill and an electric screwdriver, and an electricity storage system, an electronic apparatus, an electrically driven vehicle, an electric power system, and a control system to which the battery back is applicable.
As a power supply (secondary battery) of an electric tool, a lithium ion battery (hereinafter, appropriately referred as a “lithium ion battery”) utilizing doping and de-doping of lithium ions is used. The lithium ion battery includes a positive electrode in which a positive active material layer using a lithium composite oxide such as LiCoO2 and LiNiO2 is formed on a positive electrode current collector, and a negative electrode in which a negative active material layer using a carbon-based material such as graphite and a non-graphitized carbon material, which can dope or de-dope lithium, is formed on a negative electrode current collector. The positive electrode and the negative electrode are laminated with a separator interposed therebetween, and are bent or wound to form a battery device. This battery device is accommodated in, for example, a metal tube or a laminated film together with a nonaqueous electrolytic solution that is obtained by dissolving a lithium salt in an aprotic organic solvent to form a battery.
A protection circuit is necessary to prevent an overcharge, an overdischarge, or an overcurrent in the lithium ion battery. In the case of an electric tool, a relatively large current flows, such that a protective element with a high current capacity is necessary. As the protective element, for example, a FET (Field Effective Transistor) or a fuse is used. In addition, a technology in which a breaker is used for protection from an overcurrent, or a technology described in Japanese Unexamined Patent Application Publication No. 2005-222834 is suggested. In Japanese Unexamined Patent Application Publication No. 2005-222834, a switch having an overcurrent protection function in which a mechanical switch and a heat sensitive resistance element (Positive Temperature Coefficient: PTC element, also called PTC thermistor) are combined is described. The PTC element has a characteristic in that a resistance thereof is low at a relatively low temperature, but the resistance increases rapidly when exceeding an arbitrary temperature.
A size of the protective element such as the FET or the fuse becomes large in the case of a high current capacity, such that there is a problem in that the cost thereof increases. Furthermore, the fuse has a characteristic in that recovery is difficult, such that when the fuse is fused for the prevention of the overcurrent, the battery is protected, but reuse thereof may be difficult. In regard to the breaker, after the breaker operates once, it is necessary for a user to manually reset the breaker.
As a reusable element, a self-reset type element in which a contact returns after passage of a predetermined time from the prevention of the overcurrent and thereby discharging becomes possible is disclosed. Even in the case of using the self-reset type element, it is preferable that a safety mechanism other than the self-reset type element be constructed for securing safety of the lithium ion battery.
In addition, in the related art, it is difficult for a user to recognize a timing, at which the contact of the self-reset type element returns and thereby the discharging becomes possible, from the outside. Therefore, there is a concern that while a user is confirming the status of an electric tool in which an operation thereof has stopped at once due to the overcurrent, the contact may be returned and the electric tool may operate again.